The present application relates to the manufacture of a semiconductor device and more particularly, to a method for manufacturing a semiconductor device for forming an organic semiconductor layer by use of a dry stamping method.
Thin film transistors have wide utility as a pixel transistor in electronic circuits, especially in active-matrix, flat panel displays.
At present, most of the thin film transistors are directed to Si-based inorganic semiconductor transistors that make use of amorphous silicon or polysilicon as a semiconductor layer. The manufacture of these transistors makes use of film-forming techniques, such as a chemical vapor deposition (CVD) method. In the method, a vacuum processing chamber is necessary for the formation of a semiconductor layer, thus resulting in high process costs. In addition, thermal treatment at high temperatures is necessary, so that substrates used should have a high heat resistance.
In contrast, with thin film transistors using organic semiconductors, an organic semiconductor layer serving as an active layer can be coated and formed at low temperatures. This is advantageous in cost reduction, and film formation is enabled on a flexible substrate, such as a plastic substrate, which is poor in heat resistance.
A method of manufacturing a bottom gate/bottom contact thin film transistor using such an organic semiconductor as mentioned above is described with reference to FIG. 6. Initially, a gate insulating film 13 is formed over a substrate 11 so as to cover a gate electrode 12 formed on the substrate 11. Next, a source electrode 14 and a drain electrode 15 in pair are formed on the gate insulating film 13. Thereafter, an organic semiconductor layer 20 is formed entirely over the surface of the gate insulating film 13 so as to cover the source electrode 14 and drain electrode 15 therewith, so that a channel layer 21 made of the organic semiconductor layer 20 is formed between the source electrode 14 and the drain electrode 15. The organic semiconductor layer 20 is usually formed according to a spin coating method. A solution containing an organic semiconductor material (i.e. an organic semiconductor ink) is coated and dried.
It has been reported, for example, in Journal of Applied Physics (U.S. A.), 2005, Vol. 96, p. 5781 that with a thin film transistor of such an arrangement as mentioned above, the gate insulating film 13 is subjected to water-repellant treatment on a surface 13a thereof to form a water-repellent layer 16, for which the mobility of the organic semiconductor layer 20 formed on the gate insulating film 13 through the water-repellent layer 16 is improved, thereby increasing an on current.
It has also been reported that the organic semiconductor layer 20 is formed not by a spin coating method, but by a dry stamping method (see, for example, Journal of American Chemical Society (U.S.A.), 2004, Vol. 126, pp. 13928-13929). In this case, as shown in FIG. 7A, the surface of a transfer substrate 40 is subjected to water-repellent treatment with octadecyl trichloro silane (OTS) to form a water-repellent layer 41. It will be noted that the surface of the water-repellent layer 41 is defined as a water-repellent surface 41a of the transfer substrate 40.
Subsequently, an organic semiconductor ink is coated on the water-repellent surface 41a according to a spin coating method and dried to form an organic semiconductor layer 20. The organic semiconductor material is crystallized in contact with the water-repellent surface 41a. It will be noted that a face in contact with the water-repellent surface 41a of the organic semiconductor layer 20 is defined as a contact face 20a. Thereafter, the organic semiconductor layer 20 is thermally treated.
As shown in FIG. 7B, a stamp substrate made of poly dimethyl siloxane (PDMS) is pressed against a side of the transfer substrate 40 where the organic semiconductor layer 20 has been formed, thereby causing the organic semiconductor layer 20 to be transferred to the surface of the stamp substrate 30.
Subsequently, as shown in FIG. 7C, the stamp substrate 30 is, in turn, pressed, at a side where formed with the organic semiconductor layer 20, against the surface 13a of the gate insulating film 13 of the substrate 10 to be transferred on which the source electrode 14 and the drain electrode 15 are formed.
In this manner, as shown in FIG. 7D, the organic semiconductor layer 20 is transferred over the entire region of the surface 13a of the gate insulating film 13, with the result that the channel layer 21 made of the organic semiconductor layer 20 is formed between the source electrode 14 and the drain electrode 15. In this case, the contact face 20a that has been in contact with the water-repellent surface 41a upon crystallization of the organic semiconductor layer 20 is in contact with the surface 13a of the gate insulating film 13.
It is reported in the above-indicated Journal of American Chemical Society that the thin film transistor formed by such a transfer method as set out above works normally as a transistor.
With the thin film transistors reported in such publications as discussed above, however, an on current is not satisfactorily high, and since the organic semiconductor layer 20 is formed on the entire region over the gate insulating film 13, adjacent elements or devices are communicated with each other, with the attendant problem of an off current becomes high. This narrows an on/off ratio of the thin film transistor, thereby degrading electric characteristics.
For lowering an off current, it may occur that an organic semiconductor ink is pattern-printed according to an offset printing technique. With the offset printing, however, a hydrophilic pattern and a water-repellent pattern are, respectively, formed on a surface of a printing plate so that regions where no organic semiconductor ink is deposited are formed. In this connection, chloroform which is typical of a solvent of the organic semiconductor ink is low in boiling point and also in vapor pressure, and immediately after coating of such an organic semiconductor ink, the solvent volatilizes and thus, the organic semiconductor ink becomes thinned. This makes it difficult to form a pattern of the organic semiconductor ink.